An optical structure discussed in the present invention is a dielectric or refractive-index modulation structure in a nonlinear optical material resonantly reflecting an optical wavelength satisfying the parametric phase matching condition and the Bragg condition λm=2nΛg/m, where n is the refractive index seen by the optical wave in the material, Λg is the spatial wavelength of the dielectric modulation, and m is a positive integer. The wavelength λm is called the Bragg wavelength of order m. When the dielectric modulation is incorporated into the two ends of a laser gain medium to replace the resonator mirrors of a laser, the laser is often called a distributed Bragg reflector (DBR) laser. A DBR laser has the advantage of simplicity, because the two physical resonator mirrors are replaced by two Bragg reflectors monolithically fabricated to a laser gain medium. In a laser, a Bragg reflector can be fabricated over the entire length of a laser gain medium. The distributed optical feedback from the dielectric modulation establishes laser oscillation near the Bragg wavelength. This type of laser is termed as distributed-feedback (DFB) laser. Since different longitudinal modes in a DFB laser have different laser gain, a DFB laser has the advantage of simplicity and single-longitudinal-mode operation.
The DBR and DFB structures have been widely used in diode lasers for a variety of applications. However, the wavelength of a diode laser is limited by the energy bandgap of a semiconductor material, and cannot be varied with an arbitrary choice of the DBR or DFB dielectric-modulation period.
One way to tune a laser wavelength is to use a nonlinear optical material. Recently, advance in quasi-phase-matched (QPM) nonlinear optics (concept originally described by J. A. Armstrong et al. in Phys. Rev. 127 (1962) 1918.) has greatly enhanced the nonlinear conversion efficiency and extended the wavelength tuning range. For instance, solid-state laser pumped optical parametric generation (OPG) and oscillation from periodically-poled lithium niobate (PPLN, a kind of QPM crystal) has provided efficient and widely tunable laser sources (Myers et al. Journal of Optical Society of America B, Vol. 12 (1995) pp. 2102-2116). However, OPG starts amplification from vacuum noises and produces a broadband radiation. In the mid-infrared wavelengths, the spectral width from PPLN OPG may exceed several nanometers. To obtain efficient narrow-line laser-like radiations, QPM optical parametric oscillation is a popular means. A conventional linear-cavity optical parametric oscillator (OPO) may take advantage of the high finesse from its resonator and generate multi-longitudinal-mode laser-like radiation with a narrow linewidth in each mode. To obtain a single longitudinal mode from an OPO, one usually has to adopt a more complicated resonator design (Bosenberg et al., Applied Physics Letters, Vol. 61 (1992) pp. 378-389). Yet if one could implement a DBR or DFB structure into a nonlinear optical material for optical parametric oscillation, such an OPO is monolithic and can generate the Bragg wavelength in the parametric bandwidth. In particular, a DFB structure in the nonlinear optical medium permits single-frequency oscillation at a chosen wavelength (Yen-Chieh Huang and Yuan-Yao Lin, Journal of Optical Society of America B, Vol. 21 (2004) 777-790). A single-frequency coherent radiation source is very much desired in numerous laser applications.
It is therefore attempted by the applicant to deal with the above situation encountered with the prior art.